A flight simulator is an example of a device where the position of a movable element relative to a fixed element must be measured very accurately. While the movement in a flight simulator is rotational, other devices may generate linear displacements.
A flight simulator provides a platform supported in a gimballed arrangement, so that the platform may be rotated about two or three axes. A control system for rotation about one of the axes typically includes driving means such as a torquer, and position sensing means, such as a resolver, the output of which is fed back to control the driving means. This invention relates to improvements in the position sensing means.
The position sensing means typically includes a stator, a rotor inductively coupled to the stator, means for exciting the stator, and means for detecting the induced signal in the rotor. The relative phase between the stator and rotor signals is representative of the relative displacement between the stator and rotor. A known way of measuring the phase difference is to generate pulses at a frequency that is a fixed factor multiple of the rotor output signal frequency. The number of pulses occurring between corresponding points in the stator and rotor signal cycles, divided by the fixed factor, gives the fraction of a cycle that the two signals are out of phase.
The frequency multiplication may be accomplished by a digitally closed phase locked loop, which includes a phase detector, an integrator, a voltage controlled oscillator (hereinafter designated VCO), and a frequency divider with a fixed divisor corresponding to the multiplication factor that is to be generated. The rotor output signal provides a reference frequency which is applied to a first input of the phase detector. The phase detector generates phase error pulses which are integrated and filtered, the resultant voltage being applied to the VCO. The VCO output is applied to the frequency divider, the divider typically comprising a counter which cyclicly generates a signal when it has counted a number of pulses equal to the multiplication factor and starts counting again from zero. This signal typically occurs at the transition to zero of the most significant bit of the most significant digit of the counter. The frequency divider output is applied to a second input of the phase detector to close the loop. The output of the VCO is therefore driven to and maintained at a frequency that is the required multiple of the rotor output frequency. Moreover, the output from the counter is phase locked to a particular point in the rotor signal cycle, the particular point being in part a function of the phase detector design.
It is known in the art to provide two separate measurements of the relative rotation between the rotor and the stator, the first measurement being a "coarse" number and the second measurement being a "fine" number. Each requires a separate resolver and phase locked loop. The coarse number is capable of defining the position within the entire expected range, while the fine number defines positions within a range that is much narrower. For example, a two-pole resolver generates a 360.degree. phase shift between the stator excitation signal and the rotor output signal for every 360.degree. of relative mechanical rotation, and is suitable for extracting the coarse number. A 720-pole resolver produces an electrical phase shift of 360.degree. for every 1.degree. of relative mechanical rotation, and may be used to extract the fine number. Using a coarse number divisor of 3600 and a fine number divisor of 10,000 gives a coarse number that is generally accurate to 0.1.degree. over a 360.degree. range and a fine number that is accurate to 0.0001.degree. over a 1.degree. range.
In order to display the measured relative position, the coarse and fine numbers are displayed by latching the contents of the counters in the coarse and fine phase locked loop frequency dividers. In the above example, the coarse and fine numbers both contain digits corresponding to 0.1.degree.. This fine number tenths digit is the more accurate one. However, due to inaccuracies in the coarse resolver excitation, the coarse number should be offset with respect to the fine number. In other words, the coarse information should not be latched at the same time.
If the tenths digit of the coarse counter and the tenths digit of the fine counter at the same time were set equal, as the rotors were rotated around the full 360.degree., the tenths digit of the coarse and fine latched data would change relative to one another, with a typical error being .+-.0.2.degree.. For example, if the fine number at 359.degree. read 0.0000 and the coarse number read 358.8, latching both at the same time would give 358.0000.degree. rather than the correct 359.0000.degree.. Therefore, the tenths digit of the coarse number is purposely offset by -0.5.degree. (.+-.0.2.degree.) so that once the fine data has been latched into the display, the coarse data is allowed to "catch up" to the same number in the tenths digit as the fine latched data. The coarse data should be latched into the display once its tenth digit becomes equal to the fine data tenths digit, at which time the complete angular position data displayed is accurate.
The phase locked loop described above has certain properties that have in the past hindered the usefulness of the technique. The frequency divider in the fine number phase locked loop performs a division that is typically by a factor of 10,000 to provide in conjunction with a 720-pole resolver a precision of 0.0001.degree.. That is, the divider chain comprises a counter that counts the VCO pulses and generates the feedback signal after 10,000 pulses have occurred. Thus, the number in the counter at a fixed point in the stator cycle is, to within an additive constant, the fine number. However, when the mechanical apparatus is installed and levelled, the reading at zero angle is typically nonzero. In order to get an electrical zero to the precision of 0.0001.degree., it would be necessary to observe the counter content at the point where a zero reading is desired, and to eliminate this content by rotating the stator up to 0.5.degree. to a precision of 0.0001.degree. and bolting it down to within 0.0002 inches concentricity. As a practical matter, such an exact mechanical adjustment of the apparatus to achieve an electrical zero is impossible.
In the past, the practice has been to accept the inability to electrically zero the readout as inevitable, and to merely compensate for it in subsequent measurement and computations. As an example, the apparatus would be characterized by a fine number offset (and perhaps a coarse number offset), which would be subtracted out in order to obtain the correct angular measurements. The offsets have to maintain the relative offset of -0.5.degree. to provide a proper combination of the coarse and fine numbers.